Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of a person's arterial blood, an indicator of their oxygen supply. Oxygen saturation monitoring is crucial in critical care and surgical applications, where an insufficient blood supply can quickly lead to injury or death. FIG. 1 illustrates a conventional pulse oximetry system 100, which has a sensor 110 and a monitor 150. The sensor 110, which can be attached to an adult's finger or an infant's foot, has both red and infrared LEDs 112 and a photodiode detector 114. For a finger, the sensor is configured so that the LEDs 112 project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode 114 is positioned at the finger tip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to the assignee of the present invention and incorporated by reference herein.
Also shown in FIG. 1, the monitor 150 has LED drivers 152, a signal conditioning and digitization front-end 154, a signal processor 156, a display driver 158 and a display 159. The LED drivers 152 alternately activate the red and IR LEDs 112 and the front-end 154 conditions and digitizes the resulting current generated by the photodiode 114, which is proportional to the intensity of the detected light. The signal processor 156 inputs the conditioned photodiode signal and determines oxygen saturation based on the differential absorption by arterial blood of the two wavelengths emitted by the LEDs 112. Specifically, a ratio of detected red and infrared intensities is calculated by the signal processor 156, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The display driver 158 and associated display 159 indicate a patient's oxygen saturation, heart rate and plethysmographic waveform.